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User:Rfassbind/sandbox/CIGS structure

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Properties

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CIGS is a I-III-VI2 compound semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide, with a chemical formula of CuInxGa(1-x)Se2, where the value of x can vary from 1 (pure copper indium selenide) to 0 (pure copper gallium selenide). It is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure. The bandgap varies continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).[1]

Figure 1: Structure of a CIGS device. CdS is used optionally and some CIGS cells contain no cadmium at all.[2]

CIGS has an exceptionally high absorption coefficient of more than 105/cm for 1.5 eV and higher energy photons.[3] CIGS solar cells with efficiencies around 20% have been claimed by the National Renewable Energy Laboratory (NREL), the Swiss Federal Laboratories for Materials Science and Technology (Empa), and the German Zentrum für Sonnenenergie und Wasserstoff Forschung (ZSW), which is the record to date for any thin film solar cell.[4][5]

Structure

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The most common device structure for CIGS solar cells is shown in the diagram (see Figure 1: Structure of a CIGS device).

Substrate

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Soda-lime glass of about of 1–3 milimetres thickness is commonly used as a substrate, because the glass sheets contains sodium, which has been shown to yield a substantial open-circuit voltage increase,[6] notably through surface and grain boundary defects passivation.[7] However, many companies are also looking at lighter and more flexible substrates such as polyimide or metal foils.[8]

Back contact

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A molybdenum (Mo) metal layer is deposited (commonly by sputtering) which serves as the back contact and reflects most unabsorbed light back into the CIGS absorber.

CIGS absorber

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Following molybdenum deposition a p-type CIGS absorber layer is grown by one of several unique methods (see section Production).

Buffer

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A thin n-type buffer layer is added on top of the CIGS absorber. The buffer is typically cadmium sulfide (CdS) deposited via chemical bath deposition.

ZnO

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The buffer is overlaid with a thin, intrinsic zinc oxide layer (i-ZnO) which is capped by a thicker, aluminum (Al) doped ZnO layer.

The i-ZnO layer is used to protect the CdS and the absorber layer from sputtering damage while depositing the ZnO:Al window layer, since the latter is usually deposited by DC sputtering, known as a damaging process.[9] The Al doped ZnO serves as a transparent conducting oxide to collect and move electrons out of the cell while absorbing as little light as possible.

The heterojunction is formed between the semiconductors CIGS and ZnO, separated by a thin layer of CdS and a layer of intrinsic ZnO. The CIGS is doped p-type from intrinsic defects, while the ZnO is doped n-type to a much larger extent through the incorporation of aluminum (Al). This asymmetric doping causes the space-charge region to extend much further into the CIGS than into the ZnO.

Matched to this are the layer thicknesses and the bandgaps of the materials: the wide CIGS layer serves as absorber with a bandgap between 1.02 eV (CuInSe2) and 1.65 eV (CuGaSe2). Absorption is minimized in the upper layers, called window, by the choice of larger bandgaps: Eg,ZnO=3.2 eV and Eg,CdS=2.4 eV.

The doped ZnO also serves as front contact for current collection. Laboratory scale devices, typically 0.5 cm2 in size, are provided with a Ni/Al-grid deposited onto the front side to contact the ZnO.[10]

— to be merged with paragraph above

The CuInSe2-based materials that are of interest for photovoltaic applications include several elements from groups I, III and VI in the periodic table. These semiconductors are especially attractive for solar applications because of their high optical absorption coefficients and versatile optical and electrical characteristics, which can in principle be manipulated and tuned for a specific need in a given device.[11]

Conversion efficiency

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CIGS is mainly used in the form of polycrystalline thin films. The best efficiency achieved as of Septemberer 2014 was 21.7%.[12] A team at the National Renewable Energy Laboratory achieved 19.9%, a record at the time,[13] by modifying the CIGS surface and making it look like CIS.[14] These examples were deposited on glass, which meant the products were not mechanically flexible. In 2013, scientists at the Swiss Federal Laboratories for Materials Science and Technology developed CIGS cells on flexible polymer foils with a new record efficiency of 20.4%.[15] These display both the highest efficiency and greatest flexibility.

The U.S. National Renewable Energy Laboratory confirmed 13.8% module efficiency of a large-area (meter-square) production panel, and 13% total-area (and 14.2% aperture-area) efficiency with some production modules.[14] In September 2012 the German Manz AG presented a CIGS solar module with an efficiency of 14.6% on total module surface and 15.9% on aperture, which was produced on a mass production facility.[16] MiaSolé obtained a certified 15.7% aperture-area efficiency on a 1m2 production module,[17] and Solar Frontier claimed a 17.8% efficiency on a 900 cm2 module.[18]

Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light. The use of gallium increases the optical band gap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage.[14][19] Gallium's relative abundance, compared to indium, lowers costs.

Lab record CIGS efficiencies by substrate (a)
Substrate Glass Steel Aluminum Polymer
Efficiency 20.8% 17.7% 16.2% 20.4%
Institute ZSW (b) Empa Empa Empa
Source: Swissolar, Flisom – presentation November 2014[20]
Note: (a) lab cell with ~0.5 cm2, (b) ZSW: Zentrum für Sonnenenergie- und Wasserstoff-Forschung

Comparison

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Conventional crystalline silicon

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Unlike conventional crystalline silicon cells based on a homojunction, the structure of CIGS cells is a more complex heterojunction system. A direct bandgap material, CIGS has very strong light absorption and a layer of only 1–2 micrometers (µm) is enough to absorb most of the sunlight. By comparison, a much greater thickness of about 160–190 µm is required for crystalline silicon.

The active CIGS-layer can be deposited in a polycrystalline form directly onto molybdenum (Mo) coated on a variety of several different substrates such as glass sheets, steel bands and plastic foils made of polyimide. This uses less energy than smelting large amounts of quartz sand in electric furnaces and growing large crystals, necessary for conventional silicon cells, and thus reduces its energy payback time significantly. Also unlike crystalline silicon, these substrates can be flexible.[21]

In the highly competitive PV industry, pressure increased on CIGS manufacturers, leading to the bankruptcy of several companies, as prices for conventional silicon cells declined rapidly in recent years. However, CIGS solar cells have become as efficient as multicrystalline silicon cells—the most common type of solar cells. CISG and CdTe-PV remain the only two commercially successful thin-film technologies in a globally fast-growing PV market.

Other thin films

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In photovoltaics "thinness" generally is in reference to so-called "first generation" high-efficiency silicon cells, which are manufactured from bulk wafers hundreds of micrometers thick.[22] Thin films sacrifice some light gathering efficiency but use less material. In CIGS the efficiency tradeoff is less severe than in silicon. The record efficiencies for thin film CIGS cells are slightly lower than that of CIGS for lab-scale top performance cells. In 2008, CIGS efficiency was by far the highest compared with those achieved by other thin film technologies such as cadmium telluride photovoltaics (CdTe) or amorphous silicon (a-Si).[13] CIS and CGS solar cells offer total area efficiencies of 15.0% and 9.5%,[23] respectively. In 2015, the gap with the other thin film technologies has been closed, with record cell efficiencies in laboratories of 21.5% for CdTe (FirstSolar) and 21.7% for CIGS (ZSW). (See also NREL best research cell efficiency chart.[24])

References

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  1. ^ Tinoco, T.; Rincón, C.; Quintero, M.; Pérez, G. Sánchez (1991). "Phase Diagram and Optical Energy Gaps for CuInyGa1−ySe2 Alloys". Physica Status Solidi A. 124 (2): 427. Bibcode:1991PSSAR.124..427T. doi:10.1002/pssa.2211240206.
  2. ^ Solar-Frontier.com CIS Advantages
  3. ^ Cite error: The named reference J. Stanbery 2002 was invoked but never defined (see the help page).
  4. ^ Repins, I.; Contreras, Miguel A.; Egaas, Brian; Dehart, Clay; Scharf, John; Perkins, Craig L.; To, Bobby; Noufi, Rommel (2008). "19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor". Progress in Photovoltaics: Research and applications. 16 (3): 235. doi:10.1002/pip.822.
  5. ^ ZSW: Press Releases. Zsw-bw.de. Retrieved on 2011-09-13.
  6. ^ Hedström J., Ohlsen H., Bodegard M., Kylner A., Stolt L., Hariskos D., Ruckh M., Schock H.W. (1993). "ZnO/CdS/Cu(In,Ga)Se2 thin film solar cells with improved performance". Proceedings of 23rd IEEE Photovoltaic Specialists Conference: 364–371. doi:10.1109/PVSC.1993.347154. ISBN 0-7803-1220-1.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ Kronik L., Cahen D., Schock H.W. (1998). "Effects of sodium on polycrystalline Cu(In,Ga)Se2 and its solar cell performance". Advanced Materials. 10: 31–36. doi:10.1002/(SICI)1521-4095(199801)10:1<31::AID-ADMA31>3.0.CO;2-3.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ Dhere, Neelkanth G. (2007). "Toward GW/year of CIGS production within the next decade". Solar Energy Materials and Solar Cells. 91 (15–16): 1376. doi:10.1016/j.solmat.2007.04.003.
  9. ^ Cooray N. F.; Kushiya K., Fujimaki A.,Sugiyama I., Miura T., Okumura D., Sato M., Ooshita M. and Yamase O. (1997). "Large area ZnO films optimized for graded band-gap Cu(InGa)Se2-based thin-film mini-modules". Solar Energy Materials and Solar Cells. 49: 291–297. doi:10.1016/S0927-0248(97)00055-X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. ^ "Polycrystalline Thin Film Solar Cell Technologies" (PDF). National Renewable Energy Laboratory Colorado U.S.A. Retrieved 10 February 2011.
  11. ^ "Thin film CuInSe2/Cd(Zn)S Heterojunction Solar Cell : Characterization and Modeling", Murat Nezir Eron, PhD. Thesis, Drexel University, 1984, Philadelphia
  12. ^ http://www.zsw-bw.de/en/support/press-releases/press-detail/zsw-brings-world-record-back-to-stuttgart.html
  13. ^ a b "Characterization of 19.9%-Efficient CIGS Absorbers" (PDF). National Renewable Energy Laboratory. May 2008. Retrieved 10 February 2011.
  14. ^ a b c "The staus and future of the photovoltaics industry" (PDF). David E. Carlson Chief Scientist BP Solar 14 March 2010. Retrieved 10 February 2011.
  15. ^ "Empa takes thin film solar cells to a new level – A new world record for solar cell efficiency". Empa. 18 January 2013. Retrieved July 2015. {{cite web}}: Check date values in: |accessdate= (help)
  16. ^ Top 10 World's Most Efficient CI(G)S Modules. Solarplaza.com. Retrieved on 2013-02-18.
  17. ^ Miasole. "MiaSolé Achieves 15.7% Efficiency with Commercial-Scale CIGS Thin Film Solar Modules" (PDF). Retrieved 30 November 2012.
  18. ^ Solar Frontier,. "Solar Frontier Sets New Efficiency World Record". Retrieved 30 November 2012.{{cite web}}: CS1 maint: extra punctuation (link)
  19. ^ "Solar cell efficiency tables Ver.33" (PDF). National Institute of Advanced Industrial Science and Technology (AIST). Retrieved 10 February 2011.
  20. ^ "Flisom: Flexible PV from Lab to Fab" (PDF). Flisom AG. 4 November 2014. p. 4.
  21. ^ "First sales for 'world's cheapest solar cells'". Chemistry world February 2008. Retrieved 6 April 2011.
  22. ^ US 20090223551  patent
  23. ^ Young, D. L.; Keane, James; Duda, Anna; Abushama, Jehad A. M.; Perkins, Craig L.; Romero, Manuel; Noufi, Rommel (2003). "Improved performance in ZnO/CdS/CuGaSe2 thin-film solar cells". Progress in Photovoltaics: Research and Applications. 11 (8): 535. doi:10.1002/pip.516.
  24. ^ NREL chart of Best Research Cell Efficiencies http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
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